Portable electronic storage device such as USB flash drive has become popular in the past few years replacing other forms of storage media such as floppy disk, removable storage, CD-ROM and DVD commonly used in a computing device. Latest trend is to use flash memory as non-volatile memory (e.g., NAND flash) to build a secondary storage device such as solid state drive (SSD). SSDs do not require batteries and do not have any moving parts hence eliminating seek time, latency and other electro-mechanical delays inherent in conventional disk drives.
There are two types of flash memories today, single-level cell (SLC) flash memory and multi-bit cell (MBC) or multi-level cell (MLC) flash memory. SLC flash memory stores one-bit of data per cell, while MBC or MLC flash memory stores more than one bits (e.g., 2, 4 or other higher power of 2 bits) of data per cell. MLC flash memory has cheaper manufacturing costs than SLC does for same amount of storage or on a per MB (Mega Byte) basis.
Currently, SSDs are built using the higher cost SLC flash memory instead of MLC mainly due to one reason—relative higher data endurance (i.e., number of write/erase cycles). The problem associated with this approach is that the cost is too high for many practical applications or usages. One solution is to use MLC flash memory in a SSD. However, the data endurance rate of MLC based SSD could drop as much as a factor of ten (10) comparing to the SLC based SSD. This is not valid solution. Therefore it would be desirable to have an improved SSD device that overcomes the problems described herein.